1. Field of the Invention
This invention relates to refrigerators and freezers and more particularly refers to new and improved apparatus and method for defrosting the heat exchanger in the refrigeration chamber without exceeding the set temperature of stored material.
2. Description of the Prior Art
Refrigerating apparatus, especially a refrigerator, a freezer or the like with at least one refrigeration chamber, the set temperature of which is maintained by an electrically operated refrigerating unit controlled by a thermostat is known. A defrosting device for defrosting the heat exchanger or evaporator on the cold side can be controlled by an organ or control device, for instance, a timer, which controls the defrosting process via switching devices and associated bypass lines.
In refrigeration equipment the evaporator which serves as the heat exchanger on the cold side between the normal cooling cycles, has been defrosted at will and also automatically. In the case of automatic defrosting it is customary to control the defrosting process either dependent on the number of door openings during the operation of the refrigerating equipment or dependent on the thickness of the frost formation at the evaporator, but perhaps foremost, also dependent on time by means of a timing device. If a time-dependent control device is used, the defrosting process is controlled at fixed time intervals or periods of time dependent on the running time of the refrigeration unit.
The defrosting itself is accomplished in such known refrigerating equipment by natural incidence of heat from the outside and by supplying additional thermal energy, which is supplied either by electric heating elements such as heater resistors, heater lamps or the like, or also by reversing the refrigerant circulation in a so-called hot-gas thawing process. While the refrigeration unit is shut down during the defrosting process if electric heating energy is supplied during the thawing, it must continue to run if the evaporator is defrosted by means of hot gas, in order to supply thereby the thermal energy required for the defrosting process.
Among the many known devices and proposals for defrosting the evaporator of such refrigerating equipments, one proposal has become known, in particular, in connection with hot-gas defrosting, according to which each defrosting is to be preceded by a normal cooling cycle of the refrigeration unit. Due to the heating of the compressor, enough thermal energy is supposed to be available when the defrosting process is initiated, that the defrosting process is accelerated and an excessive temperature rise in the cold chamber is avoided. The run of the compressor which is switched on automatically, according to this proposal, prior to the defrosting however, is only one of the normal cooling cycles, which ends in the normal manner when the lower switching point of the controller is reached. The emphasis is therefore on the problem to make enough thermal energy available for the defrosting process by correspondingly heating up the compressor and the casing of the unit.
Even though the known defrosting devices work satisfactorily and reliably in customary refrigerating equipment, the operating temperature of which is generally above the freezing point, it is not directly possible to apply defrosting devices of refrigerating equipments to freezing apparatus, as in the latter, the operating temperature is far below the freezing point. The reason for this is that the danger exists here, especially because of the large temperature gradient between the evaporator to be defrosted and the freezing chamber, that the temperature in the freezing chamber rises above the permissible temperature limit because of the thermal energy supplied during the thawing process. For this reason it is not possible in freezers to equip them with the ususal defrosting devices and to stay at the same time with certainty within the requirements set for determining the quality and performance class of the freezing equipment, especially the permissible upper temperature limit for the refrigerated matter. This, however, is particularly important for the preservation and quality of the matter to be refrigerated, since an undue temperature rise during the defrosting process must be avoided at all costs in order to prevent permanent damage to the material. This circumstance must therefore be considered as the essential reason why defrosting devices are used in freezers only in rare cases.
One is therefore compelled to perform the defrosting, which is necessary in these freezers at least from time to time for them to function properly in a rather laborious manner. The recommendation given is to remove the refrigerated material from the freezer during the defrosting process and to keep it in a heat-insulating enclosure, for instance, wrapped in several layers of newsprint, and to store it in the equipment only when the defrosting is completed. In view of the effort connected with this cumbersome procedure and the warming up of the refrigerated matter which cannot be avoided in spite of great care, omitting such a defrosting device in freezers due to the difficulties mentioned means a particularly great shortcoming.
Nevertheless, in order to make regular defrosting possible also in freezers without having to tolerate a temperature rise beyond the permissible limit in the freezing chamber detrimental to the refrigerated matter, the evaporators equipped with an automatic defrosting device are arranged in known freezers outside the freezing chamber. This, however, necessitates the installation of special circulating devices, by which the air cooled at the evaporators can be circulated in the freezeing chamber via a separate system of ducts. Systems of this kind, however, are connected with a large expenditure of structural and control means and are therefore relatively expensive. In addition to particularly high energy consumption, they also have the disadvantage that they make the ratio between the external dimensions and the useful space of the freezer worse, because they are accommodated outside the freezing chamber. If the external dimensions of such freezers are limited, however, this means a considerable reduction of the useful space available.